Despite continuing research on breast cancer chemotherapy, almost all of the chemotherapeutic agents currently used to treat breast cancer still fall into just two categories: modified steroids (framework A); and compounds related to diethylstilbestrol (framework B). ##STR1##
Framework "A":
Examples of "A"-Type Chemotherapeutic Agents: ##STR2## ##STR3##
Framework "B":
Examples of "B"-Type Chemotherapeutic Agents: ##STR4##
The best agents currently in clinical use are not significantly more effective than the 30-year-old antiestrogen tamoxifen (Nolvadex.RTM.), shown above, or the first generation aromatase inhibitors. Furthermore, toxic side effects and the emergence of drug resistance continue to be problematic. It is accordingly critical that new molecular platforms be developed for the development of new, highly effective agents useful in breast cancer chemotherapy. The present invention is addressed to this timely and urgent need in the art.
The invention provides an entirely new class of compounds that are structural mimics of the two principal natural estrogens, estradiol and estrone, but are otherwise unlike any pharmaceutical agents known to date. The novel compounds are boron heterocycles that effectively inhibit human breast cancer cell growth either by blocking the estrogen-receptor-dependent growth signal, by inhibiting an estrogen-producing enzyme, or both. The compounds are also advantageous insofar as the boron atom within the molecular structure enables use in .sup.11 B nuclear magnetic resonance and magnetic resonance imaging. The novel boron-based estrogen mimics are additionally useful in boron neutron capture therapy for treating a variety of tumor types, including breast cancer. Further, those compounds of the invention that are fluorescent can be used in diagnostic procedures involving fluorescence emission spectroscopy, as will be appreciated by those skilled in the art.
Carbone derivatives of estrogens are described in Sweet (1981), "Boron Estrogens: Synthesis, Biochemical and Biological Testing of Estrone and Estradiol-17.beta., 3-carboranylmethyl Ethers," Steroids 37:223-238, where the derivative was shown to have a low affinity for the estrogen receptor of only 0.5 compared with a value of 100 for estradiol-17.beta.. Wellmann et al. (1991), "Synthesis and Biological Behavior of a Boronated Analogue of the Antiestrogen U 23,469-m," Z. Naturforsch [C] 46:252-256, concludes that the boronated analogue has a large, non-specific uptake in ZR 75-1 breast cancer cells, but does not accumulate at high enough level, in cells to have any therapeutic effect following thermal neutron irradiation. There is some description in the literature concerning boron heterocycles as potential pharmaceutical agents. For example, Groziak et al. (1994), "Boron Heterocycles Bearing a Peripheral Resemblance to Naturally-Occurring Purines: Design, Syntheses, Structures and Properties," J. Am. Chem. Soc. 116:7597-7605, discloses boron-containing purine-like heterocycles as potentially useful bioactive agents. Groziak et al. (1997), "Planar Boron Heterocycles with Nucleic Acid-Like Hydrogen-Bonding Motifs," J. Am. Chem. Soc. 119:7817-7826, pertains to similar boron-containing compounds. Robinson et al. (1998), "A 2-Alkyl Substituted 2,3,1-Benzodiazaborane," J. Acta Crystallogr. C54:71-73, relates structural information pertaining to a benzodiazaborane, specifically 1,2-dihydro-1-hydroxy-2-methyl-2,3,1-benzodiazaborine. Still other references describe 2,3,1-benzodiazaborines as antibacterial agents; see, e.g., Baldock et al. (1996), "A Mechanism of Drug Action Revealed by Structural Studies of Enoyl Reductase," Science 274:2107-2110; Grassberger et al. (1984), "Preparation and Antibacterial Activities of New 1,2,3-Diazaborine Derivatives and Analogs," J. Med. Chem. 27:947-953; and Bailey et al., "Boron-Containing Antibacterial Agents: Effects on Growth and Morphology of Bacteria Under Various Culture Conditions," J. Antimicrob. Agents Chemother. 17:549-553.
Boron-containing compounds have been used in boron neutron capture therapy (BNCT). BNCT is an anti-cancer bimodal radiation therapy utilizing a radiosensitizing compound that contains a stable boron-10 isotope and nonionizing neutron radiation. The patient is first administered the boron-containing compound that is preferably although not necessarily enriched in boron-10 isotope. In the second step, the tumor area is irradiated with thermal neutrons. Some of the neutrons are captured by boron-10 in the tumor and a nuclear reaction occurs that results in the production of helium nuclei (.alpha.-particle), lithium nuclei, and about 100 million times more energy than the initial irradiated energy. The generated energy destroys malignant cells containing the boron compounds. Selectivity is thus achieved through the use of compounds which accumulate primarily in malignant cells and/or by aiming the neutron beam at the tumor mass which contains the boron carrier. U.S. Pat. No. 5,599,796 to Schinazi et al. describes boron-containing anti-sense oligonucleotides for BNCT targeting urogenital cancer cells, while U.S. Pat. No. 5,362,732 to Spielvogel et al. describes boronate purine and pyrimidine bases and phosphate esters for use in BNCT. U.S. Pat. No. 5,612,017 to Miura et al. describes a dimeric halogenated sulfidohydroborane compound for BNCT, and U.S. Pat. No. 5,648,532 to Hawthorne et al. describes boron salts encapsulated with liposomes for use in BNCT.
The concentration of boron-10 within the tissues of patients to whom boron-containing compounds have been administered has been estimated by extrapolation from the concentration of boron-10 in blood and tissue samples. These indirect extrapolations, however, are approximate, and a more accurate method of determining the concentration and distribution of boron-10 in a patient being prepared for BNCT is needed. Multinuclear magnetic resonance imaging (MRI) and spectroscopy (MRS) are potentially valuable for evaluation of BNCT agents since boron-10 and boron-11 are magnetically active. Kabalka et al. (1988), "Boron-11 MRI and MRS of Intact Animals Infused With a Boron Neutron Capture Agent," Magnetic Resonan. Med. 8: 231-237, describe the use of cesium .mu.-disulfido-bis(undecahydro-closo-dodecaborate) in imaging experiments with rats, while the use of the compound in imaging mammals is described in Kabalka et al. (1997), J. Neuro-Oncol. 33: 153-161. In addition, French et al. (1993), "A Synthesis of 7.alpha.-substituted Estradiols: Synthesis and Biological Evaluation of a 7.alpha.-pentyl-substituted Bodipy Fluorescent Conjugate and a Fluorine-18-labeled 7.alpha.-pentylestradiol Analog," Steroids 58:157-169, describes the conjugation of a BODIPY fluorophore with an estradiol as a fluorescent probe for the estrogen receptor, while Bolger et al. (1998), "Rapid Screening of Environmental Chemicals for Estrogen Receptor Binding Capacity," Environ. Health Perspect. 106(9):551-557, describe HTS (high throughput screening) of fluorescence-based estrogen receptor ligands. Barsony et al. (1995), "Development of a Biologically Active Fluorescent-labeled Calcitriol and its Use to Study Hormone Binding to the Vitamin D Receptor," disclose the conjugation of a BODIPY fluorophore to calcitriol derivatives as a fluorescent probe for vitamin D receptor.
The use of boron heterocycles as provided herein, however, is believed to be new and completely unsuggested by the art. That is, no art of which applicants are aware discloses the boron heterocycle steroid mimics for the treatment of cancer or for any other purpose, either therapeutic or diagnostic.